By Julien Pretot

LAC DE PAYOLLE, France (Reuters) - Briton Stephen Cummings powered away on the Col d'Aspin to claim a second career Tour de France stage win as overall contender Thibaut Pinot cracked in a seventh stage that ended in chaos on Friday.

As Team Dimension Data rider Cummings celebrated his stunning solo win, back down the road the inflatable arch marking 1km to go deflated and slowed down the bunch, sending fellow Briton Adam Yates crashing.

Organisers said they were taking into account the times at the 3km mark.

Belgian Greg Van Avermaet retained the overall leader's yellow jersey at the end of the 162.5-km trek L'Isle Jourdain.

Cummings, who last year won in Mende, jumped ahead of the day's breakaway and held off 2014 Tour champion Vincenzo Nibali on the Col d'Aspin, the first big climb of this year's race, before descending safely to the Lac de Payolle to give his African-based team their fourth stage win this year.

Mark Cavendish had already won three sprints and Cummings showed again that Dimension Data could also play a role in the mountains as the Tour entered the Pyrenees.

MASK OF PAIN

"Of all my victories, I think it's the best one. The Tour is the Tour, it's special," said Cummings, who was overlooked by British Cycling for this year's Rio Olympics that will be held on a course suiting his profile.

"The Olympics is the Olympics, they made the selection. I'm over it, as an athlete you just deal with disappointment. This (the Tour) is the biggest show, it's much bigger than any Olympic Games."

In the main bunch, Pinot's FDJ team pulled in front at the foot of Aspin, a 12km ascent at an average gradient of 6.5 percent, but the Frenchman, third in the 2014 Tour and one of the main favourites, was then dropped, his face a mask of pain.

He crossed the line more than 2-1/2 minutes behind the other top guns.

South African Daryl Impey (Orica-Bike Exchange) was second and Spain's Dani Navarri (Cofidis) took third place 1:05 behind Cummings.

Cummings was part of a 29-man breakaway that split into several groups with about 30km left. He attacked with 27km remaining and never looked back.

Van Avermaet, who managed to be in the day's break, took fifth place, over a minute ahead of the top favourites, to extend his lead to 6:36 over France's Julian Alaphilippe and 6:38 over Spain's Alejandro Valverde.

Defending champion Chris Froome and last year's runner-up Nairo Quintana stayed fifth and seventh respectively, both 6:42 off the pace.

Van Avermaet, however, is unlikely to retain his lead after Saturday's eighth stage, a gruelling 184-km ride featuring three top climbs including the iconic Col du Tourmalet.

(Editing by Martyn Herman and Ken Ferris)